Long nuclear retained non-coding RNAs (IncRNAs) represent a large and relatively unmined class of RNAs that are likely to play critical roles in gene regulation and disease etiology. A major challenge is to understand the molecular functions of specific IncRNAs both at the cellular level and within the context of an organism. The long-term goal of this project is to identify and characterize IncRNAs that play a critical role in mammary gland development and the initiation and progression of breast cancer. Here, a series of Aims are presented to dissect out the role of Malati, an abundant IncRNA localized to nuclear speckles and focally amplified in a significant number of metastatic breast cancers. Studies are proposed to develop innovative loss-of-function and gain-of-function mouse models combined with cell biological approaches to assess the function of Malati in normal development and in breast cancer initiation and metastasis. The impact of alterations in the level of Malati on tissue organization will be examined, and its effect on alternative splicing in a tissue-specific manner will be pursued by next-generation RNA-sequencing analyses. Complementary cell biological studies will delve into the role that Malati plays in nuclear organization and its impact on the dynamics of pre-mRNA splicing factors enriched in nuclear speckles. The Malati RNP will be purified using an RNA-tagging strategy and its proteome will be characterized in order to identify proteins responsible for its nuclear retention and to provide additional insight into its function. In the final Aim a series of newly identified IncRNAs, that are misregulated in breast cancer, will be prioritized and several will be selected for functional analyses, to identify genes and pathways that they target, and to elucidate the mechanisms by which they contribute to breast cancer tumorigenesis. Together, the proposed studies will provide important insights into the role of several IncRNAs in normal development and cancer and will lead to new opportunities for the identification and characterization of a new and exciting class of potential therapeutic targets.